One method of determining location is through the use of satellite-based position determination systems. Satellite-based position determination systems include the U.S Global Positioning System (GPS) as well as comparable systems, such as the Russian (GLONASS), the European Galileo network, the Chinese Compass network, and the like. Such systems generally include a constellation of satellites transmitting signals that include the time the message was transmitted and, satellite position at the time of message transmission. For purposes of this disclosure, satellite navigation systems are generically referred to by the acronym GPS.
A GPS receiver calculates information about its position by precisely timing its receipt of messages from one or more of the constellation of satellites. The receiver uses the messages to determine the transit time of each message and computes the distance to each satellite using the speed of light. For example, at least four satellites can be used to determine the position of a receiver. Each satellite transmits a signal that can be used to estimate the propagation time from each satellite to the receiver. The distance between the satellite and the receiver can in turn be estimated from the propagation time. The distance from the satellite to receiver can be described in terms of the receiver position by a Euclidean distance equation. If there are four satellites, then there are four distance equations. The position of the receiver is (x,y,z,t) where t is the time offset error between the satellites and the receiver. This approach yields four equations and four unknowns
Because signals from multiple satellites are necessary for the receiver to accurately determine a position, the signal sent by each satellite in the constellation generally includes a unique coding. For example, in the U.S. Global Positioning System, a different pseudorandom noise (PRN) code is used by each satellite so that the receiver can identify the satellite from which a signal originated. The receiver identifies the satellite by cross-correlating known PRN codes with an incoming signal. Once a satellite's signal has been identified in the overlapping signals received by the receiver, the time shift of the signal is used to determine the distance between the satellite and receiver, the direct path range estimate.
The direct path range estimate between a satellite and receiver may be degraded by interference signals, which includes multipath interferers and coherent jammers. The direct path range estimate between a satellite and receiver may be degraded by incoherent signals, such as intention jamming and other signals. The type of error resulting from multipath falls into two categories, long delay and short delay. The long delay has the multipath and direct signal time delay difference being greater than the pseudo random noise (PRN) sequence chip time; the short delay is less than the chip time. The reflected signal is a copy of the direct signal that merely took a longer path to get to the receiver.
The reflected signal interfering with the direct GPS signal may cause poor tracking accuracy. GPS tracking accuracy can be particularly poor in such areas as urban environments where buildings can reflect and delay GPS signals. In some cases, reflections from buildings and other structures in some urban areas can cause GPS position accuracy to be sufficiently poor that drivers using GPS navigation can be directed off route or in circles.